C1-9-Alkanes, such as n-pentane, are a major component of the naphtha cut i.e. the gasoline fraction from petroleum refining. However, these short chain alkanes are currently not particularly well utilized in the petrochemical industry as a raw material for the production of useful chemicals.
There is considerable interest from the petrochemical industry in a process that can be applied for the upgrading and conversion of these low-cost, low-value hydrocarbon feedstock into chemicals and intermediates of higher commercial value.
Currently in the petrochemical industry, n-pentane is mostly processed by steam cracking at high temperatures to make olefins (Matar and Hatch 2000). However, this process is highly endothermic, requiring temperatures in the range of 500 to 800° C. to drive the reactions towards olefins, and is therefore very energy intensive.
Alcohols are valuable chemical products and intermediates. For example, sec-pentanols are industrially valuable chemicals that are used as solvents for paints, lacquers and varnishes, largely in the form of sec-amyl acetates (Weissermel and Arpe 1997; Lundeen and Poe 1977). They are also widely used as flavoring agents for perfumes in the fragrance industry, in the manufacture of flotation agents for non-ferrous ores in the mining industry and for the extraction of penicillin from corn-steep liquor in the pharmaceutical industry. As chemical intermediates they are used as precursors in the synthesis of other chemicals such as higher boiling esters, which find uses as plasticizers.
As such, methods to produce sec-pentanol and other short chain alcohols on an industrial scale directly from the corresponding short chain alkanes are desirable.
Secondary pentanols are currently manufactured on a large scale by the hydration of 1- and 2-pentenes with >80% concentrated sulfuric acid. The sulfuric acid pentyl esters formed are subsequently hydrolyzed to yield 2- and 3-pentanols, followed by fractional distillation for separation and purification (Lappe and Hofmann 2011; Mushenko and Dergacheva 1961). The process of hydration of olefins to secondary alcohols with sulfuric acid has been known for a long time.
One alternative process is the direct oxidation of short chain alkanes, such as n-pentane, into oxygenated products, such as alcohols and ketones. Despite the potential economic value of this approach, there are currently no practical industrial applications of this route for the conversion of short chain alkanes.
The conversion of short chain alkanes to useful chemicals by direct oxidation with molecular oxygen in the liquid phase is a significant challenge for a number of reasons. One of the main issues with the activation of short chain alkanes, is their stability and low reactivity (Shilov and Shul'pin 2000). Short chain alkanes are considerably more difficult to oxidize than longer chain alkanes as shown in Table 1, the order of reactivity correlating with decreasing C—H bond strengths as chain length increases (Teles et al. 2015; Freund et al. 1982).
TABLE 1Relative oxidation rates of different linear alkanes.n-Alkane                                 Relative          ⁢                                          ⁢          oxidation          ⁢                                          ⁢          rate                                                          Oxidation            ⁢                                                  ⁢            rate                    =                                    mol              ⁢                                                          ⁢                              O                2                                                                    (                                  mol                  ⁢                                                                          ⁢                  alkane                                )                            ×              time                                          Ethane  0.001Propane 0.1Butane 0.5Pentane 1.0Hexane 7.5Octane 200   Decane1380   
As a result of their relatively low reactivity compared to longer chain alkanes, the oxidation of short chain alkanes under typical conditions of liquid-phase reactions results in conversion rates that are too low for commercial exploitation.
Another problem is that the oxidation process is typically limited by poor selectivity to alcohols. Selectivity is challenging for two reasons.
First, the free radical autoxidation process is indiscriminate, with oxidative attack on all reactive C—H groups in the alkane molecule. Consequently, for alkanes with more than four carbon atoms such as n-pentane, a complex mixture of oxygenated products, including hydroperoxides, alcohols, ketones, carboxylic acids, and esters, with all possible isomers is formed. It is possible to obtain high selectivity, especially at low conversions, in the case of simple hydrocarbon molecules containing only one type of reactive C—H group, such as cyclohexane.
A second and more general problem with achieving high selectivity to alcohols is that the alcohols themselves are intermediate products, which are far more reactive than the alkane starting material, and are thus more readily over-oxidized into by-products such as ketones and acids. Hence the oxidation process generally offers little control over alcohol selectivity (Labinger and Bercaw 2002).
A number of approaches for the direct oxidation of longer chain alkanes (i.e. alkanes with a carbon chain length of 10 or more) to selectively give sec-alcohols are known and used industrially. These include metal catalyzed direct oxidation and direct oxidation in the presence of boron containing reagents.
The liquid-phase oxidation of hydrocarbons in the presence of boron compounds has been known for over five decades (Woods and Brotherton 1970). This approach was first developed in the late 1950s by Bashkirov et al. for the oxidation of long chain alkanes. Bashkirov showed that paraffins such as n-tridecane (C13H28) and n-hexadecane (C16H34) can be oxidized with an oxidizing gas containing 3 to 3.5% oxygen at 165 to 170° C. with 5 wt % boric acid (Bashkirov et al. 1965; Bashkirov and Kamzolkin 1959). High selectivity to alcohols was reported compared with oxidations without boric acid.
Boron promoted oxidation of alkanes received considerable attention in the 1960s and 1970s. The boron catalyzed oxidation process has been applied to the oxidation of C10-C20 alkanes for the synthesis of higher aliphatic alcohols used in the manufacture of detergents and surfactants (Griesbaum et al. 2012, Encyclopaedia of Industrial Chemistry; Arpentinier 2006, Encyclopaedia of Hydrocarbons; Weissermel and Arpe 1997, Industrial Organic Chemistry).
One commercial application that does not involve a long chain alkanes is the oxidation of cyclohexane. In cyclohexane all carbon atoms are equivalent and so cyclohexane is not representative of short chain alkanes. Further the oxidation of cyclohexane does not show high selectivity for the alcohol product. The product of the process comprises a mixture of cyclohexanol and cyclohexanone.
It is proposed that the boron containing reagent traps the alcohol when it is formed in the reaction mixture to give a borate ester. Water produced during the reaction is driven off as steam due to the high reaction temperatures employed. This effectively blocks the further oxidation from the alcohol. It is also proposed that the boron containing reagent reduces reaction rate and conversion by promoting heterolytic decomposition of the intermediate alkyl hydroperoxides.
Despite successful implementation of the boron-promoted oxidation process with long-chain alkanes and cycloalkanes, there has been no reported study of this concept applied to the oxidation of short-chain alkanes, such as n-pentane.
It is evident that the direct oxidation of short chain alkanes to alcohols with high selectivity is a difficult problem for a number of reasons such as the low reactivity and low boiling point of the alkanes. Despite approaches that have been reported for longer chain alkanes and have been known for many years, the issue of conversion and selectivity for short chain alkanes still remains largely unsolved.
The present invention aims to solve one or more of the problems associated with oxidation of short chain alkanes to alcohols such as secondary alcohols (sec-alcohols).